Avalanche Forecast
Issued: Jan 17th, 2018 10:58AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
A storm overnight will bring strong winds and new snow. The storm will cause a rapid increase in the avalanche danger overnight and through Thursday. Expect dangerous avalanche conditions. Choose conservative terrain and avoid slopes exposed to wind affected snow.
Summary
Detailed Forecast
An arriving storm overnight Wednesday will bring strong winds and snowfall. The storm should begin with rain then transition to snow overnight and through Thursday with snowfall changing to showers Thursday with diminishing winds.
The incoming storm will build increasing wind and storm slabs over a thickening crust layer. The smooth crust layer may form a potential sliding layer, allowing any avalanche releases to run long distances as wind and storm slabs build.
Building wind and storm slabs should cause dangerous avalanche conditions near and above treeline by Thursday.
Monitor new snow and wind transport carefully and choose conservative terrain.
Snowpack Discussion
Very mild temperatures Wednesday allowed for softening surface snow conditions.
Small amounts of snow fell above 6400 feet on Monday night. Breezy W-SW winds have redistributed snow and created shallow wind slabs (see observation below).
The recent high pressure created melt-freeze surface conditions followed by rain on Monday, which formed into a 2 inch thick crust at Mt. Hood Meadows, providing a good bed surface for recent snowfall. The shallow recent snow reported did not bond well to this bed surface.
Friday, a rain and freezing rain event created icy conditions reported up to at least 7000 feet.
Observations
On Tuesday, Mt. Hood Meadows Pro Patrol reported two natural 6-12" wind slabs on NE aspects between 7000 and 7300 ft.
Observations Friday from Mt Hood Meadows reported a supportable rain and freezing rain crust to 7000 feet. The ice crust prevented winds from transporting snow near and below treeline.
Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 18th, 2018 10:58AM
